Add option to set page.date automatically with the file creation date

[numist]

Adds support for a "first posted at" variable, placed in page.date, as submitted to gjtorikian/jekyll-last-modified-at by @buddhist-uni

[numist]

Currently struggling with:

  1) Date Tag A committed post file has date
     Failure/Error: post.output = Renderer.new(@site, post, @site.site_payload).run
     
     Liquid::SyntaxError:
       Liquid syntax error (line 6): Unknown tag 'page'

Removing page. from spec/fixtures/_layouts/date_with_format.html (so it's just date) doesn't change anything. I'm probably missing something obvious but it's tired and I'm late.

[numist]

On further review, I think that forking the plugin and nearly rewriting it is probably a bad plan; I don't have the bandwidth to support a plugin so I'm going to just write something checked into my blog's repo 😬

For anyone spelunking, one thing I was trying to accomplish was support for posts without dates in their filenames. Jekyll won't do this, at the moment my best thought is a generator component in the plugin that enumerates _posts for rejected files and adds them to the site's posts.